Fix link formatting for built-in functions chapter

Author: Jiggly-Balls

docs/python-tutorial/chapters/index.md

@@ -12,7 +12,7 @@ in-depth Python features.
 - [Brief Python History](01-python-history.md)
 - [Introduction to Variables](02-intro-to-variables.md)
 - [Data Types pt 1 (`string`, `int`, `float` & `bool`)](03-data-types-pt01.md)
-- [Builtin Functions pt 1 (`print` & `input` and type casting)](04-built-in-functions-pt-1.md)
+- [Builtin Functions pt 1 (`print` & `input` and type casting)](04-built-in-functions-pt01.md)
 - String Interpolation & Concatenation (f-strings, str.format & +)
 - Operators pt 1 (Comparison & Arithmetic)
 - Conditional Statements (`if`, `else` & `elif`)